Implantable medical devices are available for treating cardiac arrhythmias by delivering electrical shock therapy for cardioverting or defibrillating the heart in addition to cardiac pacing. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, senses a patient's heart rhythm and classifies the rhythm according to an arrhythmia detection scheme in order to detect episodes of tachycardia or fibrillation. Single chamber devices are available for treating either atrial arrhythmias or ventricular arrhythmias, and dual chamber devices are available for treating both atrial and ventricular arrhythmias. Arrhythmias detected may include ventricular tachycardia (VT), fast ventricular tachycardia (FVT), ventricular fibrillation (VF), atrial tachycardia (AT) and atrial fibrillation (AT).
Upon detecting an arrhythmia, the ICD delivers an appropriate therapy. Cardiac pacing is delivered in response to the absence of sensed intrinsic depolarizations, referred to as P-waves in the atrium and R-waves in the ventricle. In response to tachycardia detection, a number of tiered therapies may be delivered beginning with anti-tachycardia pacing therapies and escalating to more aggressive shock therapies until the tachycardia is terminated. Termination of a tachycardia is commonly referred to as “cardioversion.” Ventricular fibrillation (VF) is a serious life-threatening condition and is normally treated by immediately delivering high-energy shock therapy. Termination of VF is normally referred to as “defibrillation.”
In current implantable cardioverter defibrillators, the physician programs the particular anti-arrhythmia therapies into the device ahead of time, and a menu of therapies is typically provided. For example, on initial detection of an atrial or ventricular tachycardia, an anti-tachycardia pacing therapy may be selected and delivered to the chamber in which the tachycardia is diagnosed or to both chambers. On redetection of tachycardia, a more aggressive anti-tachycardia pacing therapy may be scheduled. If repeated attempts at anti-tachycardia pacing therapies fail, a higher energy cardioversion pulse may be selected.
Reliable ICD performance depends on accurate detection of arrhythmias. A delivered therapy is generally painful to the patient and depletes the battery charge. Inappropriately delivered therapies can induce arrhythmias in some patients. It is desirable, therefore, to avoid delivering a therapy due to inappropriate arrhythmia detection. For example, it is undesirable to deliver cardioversion therapy during normal, sinus tachycardia that is a heart rate increase in response to exercise. In addition, supraventricular tachycardias, which include atrial tachycardia, atrial flutter, or atrial fibrillation, may be conducted to the ventricles and detected as ventricular tachycardia or fibrillation, resulting in the delivery of a ventricular cardioversion or defibrillation therapy when no ventricular therapy may be desired.
One approach to detecting arrhythmias is based on monitoring sensed event intervals. Monitoring of sensed intervals generally involves identifying the event intervals and event rates as they occur and applying a preset group of criteria, which must be met in order to detect a particular arrhythmia. Criteria for identifying various arrhythmias may all be monitored simultaneously. An arrhythmia detection and classification system generally disclosed, in U.S. Pat. No. 5,342,402, issued to Olson et al., incorporated herein by reference in its entirety, uses criteria for sensed events, event intervals, and event rates and is employed in the Medtronic Model 7219 devices.
Certain arrhythmias may be difficult to detect based on event intervals alone. Some patients may experience ventricular tachycardia and ventricular fibrillation having similar rates or varying rates. In other cases, a high ventricular rate may in fact be due to a supraventricular arrhythmia. Criteria for arrhythmia detection may overlap. An arrhythmia detection and classification system that employs a prioritized set of inter-related rules for arrhythmia detection is generally disclosed in U.S. Pat. No. 5,545,186, issued to Olson et al., incorporated herein by reference in its entirety. The highest priority rule that is satisfied at a given time controls the behavior of the device in regard to the delivery or withholding of therapy. This methodology includes classification of sensed events into a limited number of event patterns. Certain sequences of event patterns are strongly indicative of specific types of heart rhythms. A dual-chamber Interval-based arrhythmia detection scheme of this type has been labeled PR Logic™ and is available in all Medtronic dual chamber implantable cardioverter defibrillator devices (ICDs) since introduction of the Jewel AF® and Gem DR® brand models. This interval-based algorithm generally achieves high specificity in discriminating ventricular and supraventricular arrhythmias while maintaining high sensitivity to detecting ventricular arrhythmias overall. In order to improve the specificity of the arrhythmia classification, specific criteria have been developed for effectively identifying the likely occurrence of supraventricular tachycardias and for identifying the likelihood that events sensed in the atrium are in fact far field R-waves rather than P-waves.
However, there are some arrhythmias that are known to cause detection challenges for interval based detection algorithms, such as that used by the PR Logic™ approach. The incidence of these inappropriate detections is described in an article by Wilkoff, et al., (Circulation. 2001; 103:381–386). Certain types of supraventricular tachycardias (SVTs) producing ventricular rates in the VT/VF detection zones may potentially be detected as VT or VF. One rhythm that may be inappropriately detected as VT according to interval-based detection schemes is atrial fibrillation that is rapidly conducted to the ventricles. This SVT may be detected as a double tachycardia (simultaneous ventricular and atrial tachycardia) resulting in delivery of a VT therapy.
Another example is ventricular tachycardia with long 1:1 retrograde conduction to the atria resulting in relatively regular P-R intervals that resemble a sinus tachycardia rhythm. In this case, the ventricular tachycardia may go undetected and VT therapy may be inappropriately withheld. In the reverse situation, sinus tachycardia or atrial tachycardia with long PR intervals may resemble ventricular tachycardia with 1:1 retrograde conduction, potentially resulting in inappropriate VT detection and unneeded delivery of VT therapy.
During AV nodal re-entrant tachycardia, nearly simultaneous P and R sensing may occur. When atrial sensed events occur sometimes before and sometimes after the ventricular sensed event, this rhythm might cause inappropriate VT detection. Simultaneous atrial fibrillation and polymorphic VT may have a P and R interval similar to rapidly conducted AF. Thus, this rhythm may be inappropriately classified as an SVT causing the polymorphic VT to go undetected.
An alternative approach to interval-based arrhythmia detection relies on the use of EGM morphology analysis alone to discriminate a normal EGM morphology from an abnormal EGM morphology. U.S. Pat. No. 6,393,316, issued to Gillberg et al, incorporated herein by reference in its entirety, generally discloses a method and apparatus that uses a wavelet transform to discriminate normal and aberrantly conducted depolarizations. Discrimination of QRS complexes during ventricular tachycardia from normal QRS complexes during supraventricular tachycardia may be achieved using an EGM morphology analysis. Wavelet transform analysis, as well as other morphology analysis methods, generally require greater processing time and power than interval-based detection methods. However, accuracy of morphology-based detection algorithms alone may be limited due to myopotential noise, low amplitude EGM signals, waveform alignment error, and rate-dependent aberrancy. Reference is made to Swerdlow C D, et al., J Cardiovasc Electrophysiol. 2002; 13(5):442–3.
It is recognized, therefore, that an improved system and methodology is desired to address challenges in arrhythmia detection. In particular, a method and apparatus is needed for improving the specificity of supraventricular tachycardia discrimination without compromising the sensitivity for detecting ventricular arrhythmias.